Project Summary/Abstract: Many emerging infectious diseases maintain significant populations outside of host environments. The consequences of dynamics occurring in these environmental reservoirs on the pathogenesis of these pathogens is seldom investigated and poorly characterized. This project examines how the molecular systems that facultative pathogens use to exploit hosts are shaped by the consequences of dynamics occurring within both host and environmental reservoir environments. Specifically, the researchers aim to test the hypothesis that context dependent costs and benefits result in selective dynamics within aquatic environments that lead to loss of Shigella flexneri virulence plasmid encoded virulence factors while promoting the maintenance of chromosomally encoded Shiga toxin production. The researchers will use a combination of competition experiments to measure the costs and benefits of pINV virulence plasmid carriage and virulence regulon expression in both host and aquatic environments. Further the researchers will determine how Shiga toxin expression in host and aquatic environments influences the fitness of pathogenic Shigella. Finally, the researchers will test predictions with respect to the evolutionary dynamics of both pINV encoded and Shiga toxin virulence factors using an experimental evolution approach. The researchers predict that increased duration of bouts of selection within aquatic reservoirs will result in the spread of non-invasive Shigella while the presence of Tetrahymena thermophila predation in these reservoirs will result in increased Shiga toxin production. The proposed work will determine the context dependent fitness consequences of Shigella virulence factors and determine whether selection in environmental reservoirs may contribute to the recent emergence of novel Shiga toxin producing Shigella pathogens.